1. Field of the Invention
The present invention relates to a method and apparatus, acting as a safety device, for detecting abnormal paper feeding in a sheet printer and, more particulary, for detecting abnormal paper feeding in a sheet printer from a paper feeding portion to a printing unit portion thereof.
2. Description of the Prior Art
When performing a printing operation on a rotary printing press such as shown and described in assignee's U.S. Pat. No. 5,983,796, Ishida et al., Nov. 16, 1999, an operator operates the apparatus to feed printing papers piled on a paper feeding portion to a printing unit portion to start the printing operation. However, there are times when paper feeding is not performed normally due to "an improper setting of the paper feeding portion in various kinds of apparatuses" or "problems with the papers" or the like occurs. In response to such abnormal paper feeding, various kinds of safety apparatuses are provided in a paper register portion or the paper feeding portion itself so as to automatically detect this malfunction.
For example, as disclosed in Japanese Utility Model Publication No. 2-39808, there is shown apparatus for detecting abnormal paper feeding having a luminescent element and a photoreceptor element provided near a front lay in a feeder board of the sheet printer for detecting whether an abnormal paper feeding state with respect to the front lay of a sheet paper is large or small. The apparatus includes a control circuit which transmits a cylinder disengagement signal in synchronism with an operation of the printer and a control signal for discharging sheet paper as a special state when the abnormal paper feeding state is large on the basis of a detected output from the photoreceptor element, and transmits only the control signal in synchronism with an operation of the printer when the abnormal paper feeding state is small. In accordance with the foregoing apparatus, there is an advantage if the number of interruptions of operation can be reduced and the sheet paper, in a small abnormal paper feeding state, can be discharged as the special state after being printed so as to be easily separated from a normal sheet paper.
However, conventional safety apparatus including the apparatus mentioned above performs a function as a safety device by directly detecting "a positional shift of paper" or "overlapping papers" or the like. Accordingly, the following problems can occur during a printing operation.
(1) In the situation where paper does not reach a plurality of safety devices provided in the register portion even when a timing for feeding a paper is shifted, an accurate detection and judgement as to whether the paper feeding state can or not can not performed.
(2) Generally, since the safety device performs the foregoing detection at a fixed timing (within a time period),there is a case where the detection of the abnormal paper is delayed when the timing of the fed paper is shifted, and the safety apparatus cannot perform the function as the safety device.
(3) There are times when the fed paper is interrupted by the register portion or a member provided on the feeder board, resulting in too many papers flowing into the printing unit portion, and causing damage or trouble in the apparatus or a blanket or the like.
(4) When a malfunction mentioned above is generated, it is necessary for the operator to always observe whether or not the paper fed from the paper feeding portion properly enters the register portion.
(5) When a malfunction mentioned above is generated, not only is some damage caused, but also a loss time for a post operation is generated therewith, so that not only are productivity and printing quality reduced, but also the work load for the operator is increased.